Edexcel iGCSE Biology-4.24 & 4.25 Family Pedigrees- Study Notes- New Syllabus

Family Pedigrees

🌱 Introduction

A pedigree chart is like a family tree that shows how traits or genetic conditions are inherited across generations.
Used to study patterns of inheritance (dominant, recessive, sex-linked).

📖 Key Symbols

• ♂ Square = Male
• ♀ Circle = Female
• Shaded shape = Affected individual (shows the trait/condition).
• Unshaded shape = Unaffected individual.
• Horizontal line = Mating/Parents.
• Vertical line = Children/Offspring.

🔑 Interpreting Pedigrees

1. Autosomal Dominant Inheritance

• Trait appears in every generation.
• Affected individuals usually have an affected parent.
• Males & females equally likely to inherit.
• Example: Huntington’s disease.

2. Autosomal Recessive Inheritance

• Trait can skip generations.
• Parents may be carriers (heterozygous) but unaffected.
• Both males & females equally affected.
• Example: Cystic fibrosis.

3. Sex-Linked Inheritance (X-linked recessive)

• More common in males (only one X chromosome).
• Passed from mother (carrier) to sons.
• Fathers cannot pass X-linked traits to sons (only give Y).
• Examples: Haemophilia, Colour blindness.

📊 Summary Table

📌 Quick Recap
– Shaded = affected, Unshaded = normal.
– Every generation → Dominant.
– Skips generation → Recessive.
– More males affected → X-linked.
✨ Trick: D = Dominant → Doesn’t skip. R = Recessive → Rare/hidden in some generations.